For example, in a rock drill, as shown in FIG. 2, a shank rod 2 is inserted into a front end portion of a rock drill main body 1, and a rod 22 attached with a bit 21 for drilling is connected to the shank rod 2 through a sleeve 23. When an impact piston 31 of an impact mechanism 3 of the rock drill applies impact to the shank rod 2, the impact energy is transmitted from the shank rod 2 to the bit 21 through the rod 22, and the bit 21 crushes rock R which is the object to be crushed by applying the impact.
At this time, since the reflected energy Er is transmitted from the bit 21 to the rock drill main body 1 through the rod 22 and shank rod 2, the rock drill main body 1 is once retracted by the reflected energy Er. Then after the rock drill main body 1 is advanced by thrust of a feed mechanism (not shown) by a crush length of one aplication of impact, the next impact is applied by the impact mechanism 3. The drilling work is performed by repeating this stroke.
A conventional rock drill main body 1 is provided with a chuck driver 12 as shown in FIG. 6 to rotate the shank rod 2 through a chuck 11, and a chuck driver bush 13 which abutts against a large diameter portion rear end 2b of the shank rod 2 is fitted into the chuck driver 12. This chuck driver bush 13, when thrust in a forward direction is applied to the rock drill main body 1, transmits this thrust to the shank rod 2, and at the time of impact application, the reflected energy Er from the bit 21 is also transmitted to the rock drill main body 1 from the shank rod 2 through the chuck driver bush 13.
When this reflected energy Er is directly transmitted to the rock drill main body 1 by the chuck driver bush 13, there is a fear of causing damage in the rock drill due to its impact. Accordingly, as shown in FIG. 7, some rock drills are provided with a damping piston 50 at the rear side of the chuck driver bush 13 as a buffer mechanism for buffering the reflected energy Er.
As described above, the rock drill main body 1 is once retracted after one impact application, and after it is advanced by the thrust by the crush length of one impact application, the rock drill main body 1 must perform the next application of impact. Thus, after once retracted, it is necessary to advance the rock drill main body 1 quickly by the crush length of one impact application before the next impact is applied.
When the advance is insufficient, the position of the shank rod 2 is not definite, and as shown in FIG. 8, since the bit 21 is spaced from the rock R, the impact energy of the impact piston 31 is not transmitted to the rock R, and the crushing work will not be performed. At this time, almost all the impact energy becomes the reflected energy Er and returns to the rock drill main body 1 so that not only the wear or the tools such as rod 22, bit 21, sleeve 23, etc., is increased but also a strong retracting force is exerted to the rock drill main body 1 resulting in further delay in advancement for the next application of impact.
However, usually, the strength of the reflected energy received by the hydraulic impact apparatus is different for each application of impact, and accordingly, the amount of retraction of the hydraulic impact apparatus is not uniform with large variations depending on the nature of the rock. Furthermore, a reaction force to the hydraulic impact apparatus due to the advance and retraction of the impact piston is added to the retraction force.